Enteric fevers continue to cause considerable morbidity and mortality in countries that have not yet achieved control of sewage disposal and contamination of drinking water. In these countries, the most frequent and serious cause of enteric fevers is Salmonella typhi (typhoid fever). Immunoprophylaxis against typhoid fever on a world-wide basis has not been attempted because the two presently available vaccines have limitations. The cellular typhoid vaccines induce only a limited immunity and elicit side reactions that are sufficiently frequent and sever to have discouraged their widespread acceptance. An orally administered attenuated strain of S. typhi., Ty-21a, requires three to four doses to induce about 65% protection. This vaccine is expensive, and its mode of protection has not been identified, which has prevented precise standardization of the vaccine.
Recently, two clinical evaluations, in populations with high rates of typhoid fever (about 1%/annum), have provided evidence that immunization with the capsular polysaccharide of S. typhi. (Vi) confers immunity against typhoid fever, cf. Klugman et al., Abstract 27th ICAAC, New York, N.Y., 1987; and Acharya et al., Prevention of typhoid fever in Nepal with the Vi capsular polysaccharide of Salmonella typhi, in press, 1987. The Vi vaccine, prepared under conditions which did not change its structure, elicited a four-fold or greater rise in serum antibodies in about 75% of children and adults in Nepal and in school children in the Eastern Transvaal, Republic of South Africa. The protective efficacy of the Vi in these two trials was about 70%. In contrast, the same Vi elicited a .gtoreq.four-fold antibody rise in 97% of young adults in Q France and the United States. The seroconversion rate and efficacy of other capsular polysaccharides, e.g., meningococcal vaccines, were also lower in Africa than in Finland or the United States. This lesser immunogenicity and efficacy of meningococcal vaccines was attributed to the high burden of infections, including malaria, in the African population. Since the protective response elicited by capsular polysaccharide vaccines is serum antibodies, it could be predicted that a more immunogenic Vi would be more protective against typhoid fever in high-risk populations.
Originally, Avery and Goebel in J. Exp. Med. 50:531 (1929) and Goebel in J. Exp. Med. 50:469-520 (1929) showed that the immunogenicity of pneumococcus type 3 polysaccharide could be increased by binding it chemically to a carrier protein. This principle has been applied successfully to increase the immunogenicity of capsular polysaccharides of other pathogens.
Robbins et al in J. Infect. Dis. 150, 3 436-449 (1984) disclose that a Vi polysaccharide used for immunizing humans against experimental challenge with S. typhi. failed to prevent typhoid fever. The conditions used to prepare the vaccine presumably denatured it and reduced its immunogenicity. There is no disclosure that this polysaccharide could be coupled to other proteins.
Tacket et al, in J. Infect. Dis. 154 2 342-345 (1986), disclose a vaccine made from the Vi capsular polysaccharide of Salmonella typhi, but this vaccine is not conjugated to a protein.
Miuazake et al., in Gann 71 6 766-774 (1980), disclose a method of preparing an antibody-ricin A-chain conjugate by introducing sulfhydryl groups into RAMIgG, and then reacting this modified antibody with ricin A-chain. This conjugate is said to have cytotoxicity.
Merryman et al., in Biochem. Pharmacol. 28 15 2297-2302 (1979), disclose that the proliferative response of human lymphocytes can be altered by the sulfhydryl agents D-penicillamine and 5-thiopyridoxine.
Nolan et al., in J. Clin. Microbiol. 12 22-26 (1980), disclose an assay for serum antibody to the Salmonella typhi capsular polysaccharide. There is no disclosure that this polysaccharide can be conjugated to any proteins.